The invention relates to a disc brake for a commercial vehicle having a brake caliper which engages over a brake disc, a brake-application device which is arranged in the brake caliper and which serves for pressing brake pads against the brake disc, and at least one adjustment spindle which is rotatably mounted, by way of a corresponding thread, in a bridge of the brake-application device.
In the case of a generic disc brake such as is known, for example, from DE 94 22 342 U1, a brake lever acts on a bridge which is positioned in a brake caliper and in which there are mounted two adjustment spindles, which have in each case one thrust piece for the mounting of a brake pad composed of a lining carrier plate and of a friction lining fastened thereto. The brake pad is pressed against a brake disc during a braking operation. The two adjustment spindles are provided with an external thread and are screwed in each case into a threaded bore of the bridge.
In the event of wear of the friction lining, a wear adjustment device assigned to one of the adjustment spindles serves, by rotating said adjustment spindle, to advance the brake pad in the direction of the brake disc such that an air gap between the brake pad and the brake disc is always kept substantially constant.
For safety reasons, the friction lining should not fall below a minimum thickness, wherein a so-called wear sensor is embedded, in effect as an alarm sensor, in the friction lining. A corresponding signal is triggered when the wear sensor comes into contact with the brake disc. Here, the wear sensor is positioned such that a minimum friction lining thickness remains.
In practice, however, problems often arise in that such signals are disregarded and the vehicle continues to be used, such that the friction lining becomes worn beyond the maximum admissible extent.
For design reasons, the adjustment spindles, which are guided in the bridge and which are axially adjustable relative thereto, are limited in terms of their length, such that in the event of a further advancing movement, the adjustment spindles are screwed out to such an extent as to become disconnected from the bridge. Thus, the adjustment spindles can no longer perform the actual braking function, specifically the transmission of the braking force to the brake pad, which ultimately leads to a total failure of the disc brake.
The stated problems arise not only in the event of alarm signals being ignored but also in the event of failure of the signal transducer, that is to say of the stated wear sensor.
The invention is based on the object of further developing a disc brake of the generic type such that the operational reliability thereof is improved with little outlay in terms of design and manufacture.
This and other objects are achieved by providing a disc brake having a brake caliper which engages over a brake disc, a brake-application device which is arranged in the brake caliper and which serves for pressing brake pads against the brake disc, and at least one adjustment spindle which is rotatably mounted, by way of a corresponding thread, in a bridge of the brake-application device. A wear adjustment device, which is positioned in the brake caliper, substantially compensates a wear-induced change in an air gap between the brake pad and the brake disc by way of an axial adjustment of the adjustment spindle. The adjustment spindle has, on its outer side, at least one axial groove, which is limited in terms of its length in the direction of the brake-application device and into which a stop, which is axially fixed with respect to the bridge, engages in a relatively displaceable manner.
A disc brake designed within the scope of the invention offers assurance that the adjustment spindle is not screwed out of the bridge for any reason.
For this purpose, the axial groove, which is formed into the outer side of the adjustment spindle and which is limited in terms of its length, forms a captive retention device in correspondence with a stop which is axially fixed with respect to the bridge and which engages into said axial groove.
This stop, which is arranged on that side of the bridge which faces away from the brake pad, may be formed by a positive-locking element, which may be provided in a variety of forms. Examples here are a fitted key or a ball, which is fixed to a suitable component of the wear adjustment device.
The component is preferably formed of a drive wheel which is held in an axially secured but rotatable manner on the brake caliper, specifically on the side of the brake caliper which faces toward the brake pad.
Here, the drive wheel is preferably a constituent part of a synchronizing device, which is used when two adjustment spindles are provided and arranged parallel to and spaced apart from one another, and which are to be adjusted uniformly, that is to say synchronously.
In this case, both adjustment spindles are equipped with in each case one drive wheel and are connected to one another a power transmission belt, for example a linked chain or a toothed belt. The drive wheels are then equipped with corresponding teeth.
Here, the stop which is in engagement with the axial groove performs not only the stop function but also the function of a driver for torque transmission, by which driver the force required for the wear adjustment is transmitted from one adjustment spindle to the other.
According to a further concept of the invention, provision is made for the length of the axial groove to be dimensioned such that the adjustment spindle can be screwed out only as far as a point at which a predefined minimum thickness of the friction lining, when the latter bears against the brake disc, is reached. Thus, in combination with a wear sensor, redundancy is achieved which significantly improves operational reliability.
It is also notable that the invention can be realized with little outlay, basically even in a cost-neutral fashion, if the stop is an integral constituent part of the synchronizing device.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.